1. Field of the Invention
This invention relates to mass spectroscopy and in particular to the alignment of ion optic elements in mass spectrometers. In addition, in one aspect the invention relates to electrical connections in scientific apparatus especially apparatus designed for operation in high vacuum environments.
Mass spectrometry is an analytical methodology used for quantitative elemental analysis of materials and mixtures of materials. In mass spectrometry, a sample of a material to be analyzed called an analyte is broken into particles of its constituent parts. The particles are typically molecular in size. Once produced, the analyte particles are separated by the spectrometer based on their respective masses. The separated particles are then detected and a “mass spectrum” of the material is produced. The mass spectrum is analogous to a fingerprint of the sample material being analyzed. The mass spectrum provides information about the masses and in some cases quantities of the various analyte particles that make up the sample. In particular, mass spectrometry can be used to determine the molecular weights of molecules and molecular fragments within an analyte. Additionally, mass spectrometry can identify components within the analyte based on the fragmentation pattern when the material is broken into particles. Mass spectrometry has proven to be a very powerful analytical tool in material science, chemistry and biology along with a number of other related fields.
A specific type of mass spectrometer is the time-of-flight (TOF) mass spectrometer, which analyzes ions with respect to their ratio of mass and charge. The TOF mass spectrometer (TOFMS) uses the differences in the time of flight or transit time through the spectrometer to separate and identify the analyte constituent parts. In the basic TOF mass spectrometer, particles of the analyte are produced and ionized by an ion source. The analyte ions are then introduced into an ion accelerator that subjects the ions to an electric field. The electric field accelerates the analyte ions and launches them into a drift tube or drift region. After being accelerated, the analyte ions are allowed to drift in the absence of the accelerating electric field until they strike an ion detector at the end of the drift region. The drift velocity of a given analyte ion is a function of both the mass and the charge of the ion. Therefore, if the analyte ions are produced having the same charge, ions of different masses will have different drift velocities upon exiting the accelerator and, in turn, will arrive at the detector at different points in time. The differential transit time or differential ‘time-of-flight’ separates the analyte ions by mass and enables the detection of the individual analyte particle types present in the sample.
In a time of flight mass spectrometer (TOFMS), the ion accelerator accepts a stream of ions from an ion source and accelerates the analyte ions by applying an electric field. The velocity of a given ion when it exits the ion accelerator is proportional to the square root of the accelerating field strength, the square root of the charge of the ion, and inversely proportional to the square root of the mass of the ion. Thus, ions with the same charge but differing masses are accelerated to differing velocities by the ion accelerator.
When an analyte ion strikes the detector, the detector generates a signal. The time at which the signal is generated by the detector is used to determine the mass of the particle. In addition, for many detector types, the strength of the signal produced by the detector is proportional to the quantity of the ions striking it at a given point in time. Therefore, the quantity of particles of a given mass can also often be determined. With this information about particle mass and quantity, a mass spectrum can be computed and the composition of the analyte can be inferred.
In a typical linear TOF-MS, as described, for example, by Wiley and McLaren (Rev. Sci. Instrum. (1955) 26:1150-1157) and in U.S. Pat. No. 2,685,035, ions are accelerated in vacuum by means of electrical potentials. The potentials are applied to a set of parallel, substantially planar electrodes, which have openings that may be covered by fine meshes to assure homogeneous electrical fields, while allowing the transmission of the ions. The direction of the instrument axis is usually defined as the direction normal to the flat surface of these electrodes. Following the acceleration by the electrical fields between the accelerator electrodes, the ions drift through a field free space of a flight tube until they reach the essentially flat surface of an ion detector. At the detector or detector surface, the arrival of the ions is converted in a way to generate electrical signals, which can be recorded by an electronic timing device. An example of such a detector is multi-channel electron multiplier plate. The measured flight time of any given ion through the instrument is related to the ion's mass to charge ratio.
In another typical arrangement such as, for example, that disclosed in U.S. Pat. No. 4,072,862, the motion of the ions is turned around after a first field free drift space in a flight tube by means of an ion reflector. This arrangement is generally referred to as reflector or reflectron TOF-MS. In this approach the ions reach the detector after passing through a second field free drift space in a flight tube. The properties of such ion reflectors allow one to increase the total flight time, while maintaining a narrow distribution of arrival times for ions of a given mass to charge ratio. Thus, mass resolution is enhanced over that of a linear instrument.
Extraction of ions from molecular beams has also been applied to TOF-MS. In one such approach often referred to as orthogonal accelerated TOF-MS, molecular beams can be produced by expansion of gas from a high-pressure region to a vacuum through two or more orifices separating the regions. The molecular beam may contain ions that were formed in the expanding gas or neutrals in the beam can be ionized by interaction with ionizing radiation. A packet of ions can be extracted from a section of the beam by momentary application of an electric field at right angles to the beam. The time of flight over a distance perpendicular or substantially perpendicular to the axis of the molecular beam can be measured from the instant that the extraction field was turned. One such approach is described by O'Halloran, et al., Technical Documentary Report No. ASD-TDR-62-844, April 1964. This approach utilizes a drift tube oriented at 90 degrees to receive the ion packet. Steering electrodes are generally employed to deflect the ion packet at various angles at or near the perpendicular depending on the nature of and presence of a drift tube.
In the construction of TOF-MS spectrometers, whether linear, orthogonal accelerated or the like, the alignment of the individual components is important to achieve high levels of resolution of the spectra peaks. In all TOF-MS it is important to keep the source and the detector ion elements parallel to each other within fractions of a degree to achieve acceptable resolution. Additionally, for orthogonal accelerated TOFMS it is also important to maintain perpendicularity between the ion source and the pulsing optics.
Typical solutions to achieve requisite alignment involve attaching the ion optic elements to ends of a tube that is carefully constructed or to the surface of a flat plate. These approaches require painstaking adjustments to achieve sufficient alignment. The known solutions suffer from poor resolution due to lack of alignment accuracy in the ion optics components. Another disadvantage of the known approaches is that they require the difficult and time-consuming process of aligning the ion optics elements.
Another consideration in scientific apparatus designed for operation in high vacuum environments is the need for a means to make electrical connections from one element to another such as, for example, an input connector pin to an electrode, without upsetting the desired electrical fields in the vicinity of charged particle beams. This situation is particularly of importance in mass spectrometry. Typical solutions involve the use of shielded conductors made from braided wire or solid-walled tubular covers over polymer insulators or planar shields and partitions made from sheet metal.
The typical solutions to the above problem suffer from the difficulty of making high vacuum and high temperature compatible shielded conductors using the above methods. When accomplished, the result is usually expensive, delicate and inflexible. Additionally, if a constant electrical impedance is desired, a solid dielectric material and conductor support may be needed, which leads to outgassing of the dielectric material in the high vacuum systems.
2. Brief Description of Related Art
A discussion of designs for mounting optical components is found in “Building Scientific Apparatus, A Practical Guide to Design and Construction,” Second Edition, Addison-Wesley Publishing Company, Inc., Redwood City, Calif., 1989, pages 170-177 and 336-337. Various optical rails, carriers, clamps, blocks, adaptors, translators and holders are also known for mounting optical components. However, there is still a need in mass spectrometry for the alignment of individual components of the ion optics system to achieve high levels of resolution of spectral peaks.
One embodiment of the present invention is an apparatus comprising a base having a front face, a rear face and at least one side face, and at least two supports. Each of the supports has at least one face. Each of the supports is affixed to the base by alignment of a portion of at least one face of the base and a portion of at least one face of the support thereby resulting in the alignment of the supports relative to one another. At least one of the supports has attached thereto a component of an ion optics system for a mass spectrometer.
In another embodiment of the present invention the apparatus has at least one groove therein. An electrical lead is sequestered in the groove and the apparatus further comprises a shielding plate covering the groove.
Another embodiment of the present invention is a mass spectroscopy apparatus comprising components of an ion optics system for a mass spectrometer affixed to a mounting base. Each of the components is affixed to a support. Each of the supports has at least one support mating face. The mounting base comprises a plurality of base mating faces respectively corresponding to a respective support mating face. The support mating faces and the base mating faces are configured and dimensioned such that, when the support mating faces are brought together in registration with the respective base mating faces, the components are optically aligned within acceptable tolerances.
Another embodiment of the present invention is a method for constructing an apparatus comprising a plurality of components of an ion optical system for a mass spectrometer. The method comprises bringing together (i) a base having a front face, a rear face and at least one side face, and (ii) a plurality of supports. Each of the supports has at least one face and is attached or is attachable to one of the supports. A portion of a face of each of the supports is aligned with a corresponding portion of at least one face of the base. The portions are secured to one another. The components of the optical system for a mass spectrometer are affixed to the supports prior to or subsequent to securing the portions to one another. The portions of the faces are configured and dimensioned such that, when the portions are secured, the components are optically aligned within acceptable tolerances.
Another embodiment of the present invention is a method of constructing a mass spectroscopy apparatus comprising components of an ion optics system. Each of the components of an ion optics system for a mass spectrometer is affixed to a mounting base. Each of the components is affixed to a support either prior to or after the support is affixed to the mounting base. Each of the supports has at least one support mating face. The mounting base comprises a plurality of base mating faces respectively at corresponding to a respective support mating face. The support mating faces and the base mating faces are configured and dimensioned such that, when the support mating faces are brought together in registration with the respective base mating faces, the components are optically aligned within acceptable tolerances. The mounting base is secured to a frame of the mass spectroscopy apparatus.
Another embodiment of the present invention is a scientific apparatus for use in high vacuum environments. At least one electrical connection in the apparatus is made by means of a base having a groove in at least one face thereof wherein an electrical lead is sequestered in the groove and wherein a shielding plate covers the groove.